The concept of having a 'fuel gauge' indication for a Lipo depends on your expectation of accuracy and context. A Lipo single cell has a range of output voltages that vary pretty linearly from 4.2vdc at 100% of charge to 3.0 at what most consider a good 0% or empty charge. Discharging a lipo cell too low can permanently damage the cell so different people use different 'cut-off' voltage value. Now if you wanted a totally accurate fuel gauge measurement value you would not depend just on the cells terminal voltage but rather measure the actual mAH consumption of current consumed over time, which is a lot more complex then just measuring the terminal voltage of the cell. And of course if your specific lipo battery is made from a number of series wired single cells then the voltage values I gave above would be 2X or 3X, etc depending on the number of series cells your pack has.

So if your dealing with just a single cell then wiring the cells output terminal to an arduino analog input pin would allow you to measure the cell's terminal voltage and then based on that value calculate it's relative position between 0% and 100% terminal voltage. If your using a multiple cell lipo then you would need to use a two resistor voltage divider to safely measure the voltage on your arduino and the scale as required in your software.